There are many Twitter apps for the iPad already, but my favorite is Twitterific. The interface is really simple, intuitive, and is just all-around good looking. In portrait mode, all you see is your time line, where you can easily reply, retweet, or mark a tweet as favorite. Flip it to landscape mode, and you'll see a sidebar on the left that leads you to your Replies, Lists, and more. The free version has ads at the top, but they're not that obtrusive.